An organic EL element in which an organic light-emitting layer is provided between a plurality of electrode layers to electrically cause light emission has been studied as to its use as a display element which can be an alternative to a liquid crystal cell. Further, the use of an organic EL device as a surface light source device, such as a flat-type lighting device or a backlight for a liquid crystal display device, taking advantage of the characteristics such as a high luminous efficiency, low driving voltage, light weight, and low cost is also being studied.
One issue when using an organic EL element as a light source for a surface light source device is how to efficiently extract light in a useful form from the element. For example, although the light-emitting layer of the organic EL element itself has a high luminous efficiency, during passage of the light through the layered structure constituting the element until light emission therefrom, the light amount decays due to, e.g., interference in the layers. Thus, such loss of light needs to be reduced as much as possible.
As a method for increasing light extraction efficiency, it has been known that various concavo-convex structures are provided on a light extraction surface side of a light source device (FIG. 4, FIG. 6, and the like in Patent Literature 1, for example). As a component of the organic EL element having such a concavo-convex structure, there has been known an optical sheet including a layer on which a concavo-convex structure is formed using what is called a photopolymer method (2P method).